Citric Acid-Assisted Accumulation of Ni, Cr, and Co by Maize Successively Grown in a Tropical Ultramafic Soil

Phytomining of ultramafic soils promises to turn these unfertile lands into economically viable alternatives to metal extraction. The technique relies on plants that concentrate exceptionally elevated metal concentrations in shoots and produce sufficient aerial biomass. Due to the limitations of the low biomass of most hyperaccumulators, we tested applying citric acid to assist the accumulation of Ni, Co, and Cr by maize plants grown on ultramafic soil. In addition to studying plant metal accumulation resulting from citric acid application to the soil, we also assessed how the chelator affects the distribution of metals into soil fractions. The treatments consisted of five rates of citric acid (0, 5, 10, 20, and 40 mmol kg −1 of soil). Three successive cultivations of 40 days each were carried out. Applying 20 mmol kg −1 of citric acid was the most effective for assisting the phytoextraction of Ni, Cr, and Co. Iron oxides were the main pools for Ni allocation. On the other hand, manganese oxides had a high affinity for Co, and Cr was mainly retained in the residual fraction. Citric acid effectively remobilized these metals to the soil exchangeable fraction. Induced phytomining using maize was not an alternative to hyperaccumulators for the conditions of this study since the Ni recovered would be far from adequate to make the technique economically feasible. On the other hand, the solubilization and consequent increase in metal accumulation demonstrated here confirm the potential of using citric acid in soil remediation using non-accumulating high biomass plants.